Two decades ago, it was found that gold is catalytically active as clusters although
it is known to be inert as bulk material. In the work presented in this thesis, density
functional theory calculations have been applied to show that the properties of gold
clusters and small molecules, namely O2 and NO2, are di erent on oxide thin lms
supported by metal carriers than on single crystal oxide surfaces. Collectively, these
di erences are called thin- lm e ects. The e ects and the mechanisms behind them
have been studied with atomistic calculations and rationalized with simple physical
models. The signi cance of the thin- lm e ects for heterogeneous catalysis is discussed.
It has been found that on MgO/Mo thin lm, the adsorption of gold clusters is strong,
whereas on single crystal MgO, the adsorption is weak. On MgO/Mo and MgO/Ag,
the clusters are charged and tend to maximize their surface contact whereas on single
crystal MgO, they are essentially neutral and the smallest clusters have only few
atoms in contact with the surface. As the clusters are charged, their adsorption energy
correlates to the electron a nity of gas-phase clusters. The tiniest Au2&#8722;6 favor
chain-like structures on MgO/Mo. The larger clusters of around 7-20 atoms are two
dimensional and planar on MgO/Ag. These structures are not favored by the clusters
adsorbed on single crystal MgO. Furthermore, also O2 and NO2 become charged on
the studied thin- lms.
In addition to electron a nity of the adsorbate, the strength of the thin- lm e ects
seem to depend on oxide lm thickness and the work function of the substrate. The
long-ranged character of the e ect can be addressed to image charge and polarization
interaction. It has been predicted that the thin- lm e ects are signi cant when the
lm thickness is less than 1-2 nm.
The electronic states of the chain and planar clusters can be rationalized in the light
of one or two dimensional harmonic oscillator model depending on the cluster size.
E ectively, a cluster can be seen as a harmonic potential to which the 6s valence
electrons of gold atoms and the charge transferred from the substrate are con ned.
By applying these ideas, it has been possible to identify experimentally observed
clusters in a combined scanning tunneling microscopy and density functional theory
study. This is an important step in characterizing thin- lm based cluster catalytic
model systems.
Calculations of O2 adsorption on Au1&#8722;6/MgO/Mo show that O2 tends to stick at the
ends of the chains. However, adsorption to bare MgO/Mo terrace is slightly more
favorable. Based on these results, one can suggest a CO oxidation scheme in which
Au traps CO molecules whereas O2 sticks on the terraces. As the surface di usion
barrier of O2 is low, the molecules occasionally reach Au clusters with CO and the
CO2 is released on the cluster boundary. It should be noted, however, that also other reaction pathways can be possible and they may depend on the carrier metal and the
cluster size.

en

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eng

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University of Jyväskylä

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Research report / Department of Physics, University of Jyväskylä;0075-465X ;no. 15/2009